1. Field of the Invention
The present invention relates to a magneto-optical recording medium using an exchange-coupled, multilayered magnetic film and capable of direct overwriting by light power modulation, and to a method of manufacturing the same.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing the structure of a magneto-optical recording medium of prior art, such as disclosed in "Journal of Applied Physics," vol 67, No. 9, pp. 4415-4416, May 1, 1990. In the figure, the reference numeral 10 is a light beam emitted from a semiconductor laser or the like and focused by a lens; 20 is an external magnetic field generator for generating a biasing magnetic field Hb; 30 is a transparent substrate made of glass or plastic material; 1 to 5 are first to fifth magnetic layers exchange-coupled with one another; and 6 and 7 are dielectric layers. In the magnetic layers 1 to 5, adjacent layers are coupled with each other by the exchange forces acting between them, i.e., the forces by which the directions of magnetization (sub-lattice magnetization) of the rare earth or transition metals in the magnetic layers tend to align in parallel with each other.
The mechanism of direct overwriting by light power modulation will now be described. FIG. 2 is a diagram showing the intensity of the light beam 10 which is modulated in three stages for writing P.sub.HIGH, P.sub.LOW) and reading (P.sub.READ). In P.sub.READ, the light beam intensity is small and the magnetization states of the magnetic layers do not change. On the other hand, in P.sub.LOW and P.sub.HIGH, the light beam intensity is large compared to that for P.sub.READ, so that the temperature of the recording medium reaches maximum values T.sub.LOW and T.sub.HIGH, respectively; during the subsequent cooling, the magnetization state changes, recording a "1" or a "0". Zeros are recorded by "low power process", while ones are recorded by "high power process".
FIG. 3 is a schematic diagram showing the low power process and high power process in relation to the temperature of the recording medium and the direction of transition metal sub-lattice magnetization of each magnetic layer. In the figure, Troom indicates room temperature, and T.sub.c1, T.sub.c2, T.sub.c3, and T.sub.c4 are the Curie temperatures for the first to fourth magnetic layers 1 to 4, respectively. After forming the film on the recording medium, the directions of transition metal sub-lattice magnetization of the second to fourth magnetic layers 2 to 4 are oriented upward only once at the beginning, and the biasing magnetic field Hb is formed downward. In the figure, the arrows indicate the directions of transition metal sub-lattice magnetization.
When a light beam is projected in the low power process, the temperature of the medium rises to T.sub.LOW which is in the vicinity of T.sub.c1. At this time, there occurs no change in the directions of transition metal sub-lattice magnetization of the second and fourth magnetic layers 2 and 4, the directions remaining oriented upward. During the sub-sequent cooling, the direction of transition metal sub-lattice magnetization of the first magnetic layer 1 is aligned upward by the exchange forces operating from the second magnetic layer 2, thus recording a "0".
On the other hand, when a light beam is projected in the high power process, the medium temperature rises to T.sub.HIGH which is in the vicinity of T.sub.c2. At this time, the direction of transition metal sub-lattice magnetization is oriented upward only in the fourth magnetic layer 4. During the subsequent cooling, the direction of transition metal sub-lattice magnetization of the second magnetic layer 2 is oriented downward because of the magnetic field Hb generated by the external magnetic field generator 20, and then, the direction of transition metal sub-lattice magnetization of the first magnetic layer 1 is aligned downward by the exchange forces operating from the second magnetic layer 2, as in the case of the low power process. Thereafter, near the room temperature, the direction of transition metal sub-lattice magnetization of the second magnetic layer 2 is aligned upward by the exchange forces operating from the third magnetic layer 3, the so-called initialization process. However, no reversal by exchange forces occurs in the transition metal sub-lattice magnetization of the first magnetic layer 1, so that the magnetization direction remains oriented downward, thus recording a "1".
Next, the initialization process of aligning the magnetization of the second magnetic layer 2 back into the initial upwardly oriented state will be described in further detail. During the cooling from the high power process, the direction of magnetization of the third magnetic layer 3 is aligned with the direction of magnetization of the fourth magnetic layer 4 because of exchange forces, thus upwardly orienting the transition metal sub-lattice magnetization of the third magnetic layer 3.
Next, the direction of magnetization of the second magnetic layer 2 is aligned with the direction of magnetization of the third magnetic layer 3 because of exchange forces, thus upwardly orienting the transition metal sub-lattice magnetization of the second magnetic layer 2 to return to its initial state. The initialization process is thus completed. Since the fourth magnetic layer 4 works always to initialize the direction of magnetization of the second magnetic layer 2 to upward orientation to ready it for the next recording, the fourth magnetic layer 4 is also called an initializing layer.
As described above, to realize direct overwriting, the magnetization of the initializing layer 4 must always be retained in the same orientation and must not be reversed in any write or read process including the high power process. However, it has been found that the direction of magnetization of the initializing layer 4 may become reversed when the light power P.sub.HIGH in the high power process is excessively high.
FIG. 4 shows the dependence of the carrier-to-noise (C/N) ratio of a reproduced signal on the power P.sub.HIGH of a recording light beam. As can be seen, when P.sub.HIGH exceeds 10 mW, the C/N ratio abruptly drops. When the initializing layer 4 was observed under a polarizing microscope, it was found that there had occurred magnetization reversal in part of the initializing layer in the recording area. This was because the magnetic layer temperature had risen nearly to the Curie temperature for the initializing layer due to excessive P.sub.HIGH, causing magnetization reversal in the initializing layer 4.
FIG. 5 is a diagram showing the bit error rate of a reproduced signal plotted against the record/reproduce repetition times when recording and reproduction processes were repeatedly performed. As can be seen, the bit error rate shows an increase when the record/reproduce repetition times approximately exceeds 1.times.10.sup.4. When the magneto-optical recording medium with an increased bit error rate was examined, it was found that magnetization reversal had occurred in the initializing layer 4. It is considered that the reversal or fluctuation of the magnetization direction of the initializing layer 4, which would present no problem in one recording/reproduction, was exaggerated by repeated recording and reproduction to a point where such magnetization reversal occurred in the initializing layer 4 as to lead to an increase in the error rate.
As described above, with the magneto-optical recording medium of FIG. 1, it has been found that the upper limit of the recording light beam power has to be set at a relatively low value, and in some applications, there is a limit to the number of record/reproduce repetition times, because of the problem of magnetization reversal of the initializing layer 4 inherent to the light-modulation direct overwritable magneto-optical recording medium.
The four magnetic layers used in the above recording medium have the magnetic characteristics and film thicknesses shown in Table 1.
TABLE 1 ______________________________________ Curie Film Dominant temper- thick- Film Coercivity sub- ature ness material (KOe) lattice (.degree.C.) (.ANG.) ______________________________________ 1st TbFeCo 15 TM 200 600 layer 2nd GdDyFeCo 3 RE 260 1400 layer 3rd TbFe 3 TM 120 200 layer 4th TbCo 8 RE &gt;300 400 layer ______________________________________
As shown in Table 1, the total film thickness is 2600 .ANG., and TM means that the transition metal sub-lattice magnetization is greater than the rare earth metal sub-lattice magnetization, while RE means that the rare earth metal sub-lattice magnetization is greater than the transition metal sub-lattice magnetization.
Thus, the prior art magneto-optical disk has the problem that because of the large total thickness of the magnetic layers means, a large laser light power is required for recording, resulting in reduced recording density. The prior art has the further problem that the complicated mechanism imposes a limit to the allowable range of the external magnetic field to be applied when overwriting, making it difficult to obtain a sufficient signal-to-noise ratio.
FIG. 6 is a schematic diagram showing the structure of a prior art magneto-optical recording medium, as disclosed in Japanese Patent Application No.2-121103, which has a magnetic layer for controlling the exchange forces. As shown, the recording medium has a fifth magnetic layer 15 interposed between the first and second magnetic layers 1 and 2, the fifth layer 15 being formed from a rare earth dominant metal film and having the ability to control the exchange forces. When performing direct overwriting by light power modulation on the recording medium, since the direction of sub-lattice magnetization of the second magnetic layer 2 must be prealigned in a prescribed direction, an auxiliary magnetic field generator 40 capable of generating a magnetic field as great as several kOe is needed for initialization, in addition to the external magnetic field generator 20.
In the above magneto-optical recording medium, in order to reduce the auxiliary magnetic field from the auxiliary magnetic force generator 40, which causes a problem that the system becomes complicated in construction, the thickness of the second magnetic layer 2 should be reduced. Reducing the thickness of the second layer 2 results in reduction of the allowable margin for the formation of the first and second magnetic layers 1 and 2, causing the problem that stable production is difficult.
The above prior art recording medium has the further problem that since no particular limits are imposed on the reversal magnetic field for the first magnetic layer 1 as long as it is within the range that permits retention of recorded information, good recording and reproducing characteristics cannot be obtained when the reversal magnetic field for the first magnetic layer 1 is extremely small.